On Fri, 11 Nov 2005, Vincent Diepeveen wrote:
Vincent, there is so much to comment on here that I don't know where to
begin.
First of all, 50Hz just doesn't (much) radiate. There is a factor that
goes like the size of the antenna divided by the wavelength squared, and
the wavelength at 50 Hz is, um, "large" -- a tenth of a light-second or
so, compared to a tenth of a meter or so. Squared. "Static"
(time-varying at 50 Hz, but non-radiating) fields probably exist around
isolated AC wires carrying large currents, none of which will be
anywhere near the systems, since by hypothesis they are being run by a
power supply some distance away and run on DC.
Second, EM >>radiation<< (including both of the E and M fields it is
made up of) is strongly excluded from most conductors, with the
nonlinear exception of ferromagnetic metals for static magnetic fields.
Metals are dielectric and diamagnetic in that they expel field. An EM
wave incident on a piece of metal is typically absorbed and reflected
within a "skin depth" of the surface that is again a very small number.
A piece of aluminum foil is plenty adequate to completely screen most EM
radiation; so is aluminum screen (like that used in certain windows)
provided that the hole size is smaller than the wavelength. Microwaves
carrying quite a lot of power (enough to cook you) are easily screened
by a simple sheet of metal punched with holes in it across the window so
you can see into it without frying your eyes or brain (although being
paranoid I don't stand with my eyes pressed up to the glass watching my
popcorn pop:-).
Third, the cancer scare is just plain that -- a scare. There is no
statistically significant evidence that living under or near high
voltage power lines causes cancer. The only time such evidence WAS
reported, the statistics of the study was questionable, and reanalysis
completely eliminated the hypothesized effect. Nor is there ANY
credible mechanism for them to cause cancer -- the power available in
the fields produced by high voltage lines is simply not adequate to
EITHER raise the temperature of anything a measurable fraction of a
degree (one doesn't exactly cook one's thanksgiving turkey by going
outside and setting it under the high voltage power line) OR to cause
e.g. quantum mechanical transitions with enough energy to damage
molecules such as DNA. Finally, such radiation as there is has a
penetration depth that is tiny into any conductor and wait! We are
conductors! Just as is the case with sunlight, the only part of our
bodies that are actually EXPOSED to radiation fields from power lines is
our skins, but melanoma was not the cancer reported.
Then (to quote http://en.wikipedia.org/wiki/Earth's_magnetic_field):
The strength of the field at the Earth's surface at this time ranges
from less than 30 microteslas (0.3 gauss) in an area including most
of South America and South Africa to over 60 microteslas (0.6 gauss)
around the magnetic poles in northern Canada and south of Australia,
and in part of Siberia.
so if (to quote you):
> Please note that general agreement seems to be that it is wise to avoid new
> situations where persons must nonstop live in a field above 0.4 microtesla.
then the human race is SOL, isn't it. And here is our government, doing
nothing about it! Time to migrate to the moon or something except that
wait! The Earth's field, a hundred times bigger than "general
agreement" says is safe, helps to shield us from the very radiation
streaming at us from the Sun that that IS ionizing (molecular bond
breaking) and hence IS likely to cause cancer. I'd worry more about
what will happen when the Earth's magnetic field vanishes, as it
appearst to be doing at a possibly alarming rate, or reverses, as it
does every quarter-million years or so.
Finally, you seem to completely misunderstand the difference between
volts and amperes. If one places one's head up against a 400000 volt
overhead primary supply wire there is indeed a health risk but it ain't
from magnetic fields. I'll have to pass this one on to my
brother-in-law the electrician, he'll find it a hoot. It is from the
blinding searing pain you'd experience in the fraction of a second that
your nervous system still "functioned" while it exploded into a plasma,
driven by a more or less electroSTATIC process (as 50-60 Hz is, as
noted, a "slowly varying field" and you're in the NEAR field zone where
radiation is irrelevant except, of course, for the high frequency
fourier components of the lightning bolt that blows you apart.
Magnetic fields are produced by CURRENT, not voltage. Two wires 1 meter
long carrying a current of 1 ampere and separated by one meter produce a
force of 2x10^-7 Newtons on one another. Since the magnetic field
constant DEFINES the ampere, the magnetic field one meter away from a
long straight wire carrying one ampere of current is EXACTLY 2x10^-7
Tesla (or 2 milligauss). It varies like 1/r as one approaches the wire,
so at order of a millimeter it is order of a gauss. The maximum
currents inside a PC are order of 10 amperes (depending on voltage and
power drawn on the different DC supply lines and they tend to be cm away
from things, so their fields too probably approach a whole gauss. This
is sensible -- Gauss are units we use as they are RELEVANT to everyday
fields where a field of 1 Tesla is so large that it actually IS
dangerous (at least to people wearing iron or other ferromagnetic metal
or carrying around bank cards in their wallet -- I've trashed two of the
latter carrying around demonstration magnets in the department:-(. Note
well that this large a field is still a factor of 40 or smaller than the
Earth's magnetic field.
The only statement below that I unambiguously agree with is the one
about amateuristic methods.
So, for the record, the EM shielding I was referring to is that that
might be required for radiation in the GHz range, caused by bleedout
from the CPU clock or other system oscillators, from motherboard traces
or unshielded lines that are on the order of cm in length (since 3 GHz
radiation has a wavelength of around 0.1 meter and will radiate
significantly from an antenna a few cm in size and can be picked UP
efficiently by other nearby antenna a few cm in size).
My original (64K motherboard) IBM PC radiated like an SOB. I could tell
because I could literally hear it as interference on the radio I had in
my office when I tuned to the right band of frequencies. The problem
persisted through the AT, but then all but disappeared. You can still
"hear" a bit of bleed-through radiation if you listen to music on a
computer and have a good ear, sometimes, but they engineer motherboards
to AVOID the problem and largely succeed. In no case was the RADIATION
dangerous. It is just within the realm of possibility that if you mount
two cheap motherboards too close to one another you'll induce enough
voltage in certain frequencies between them to excite e.g. a bit flip
and ruin your computation(s) or cause system crashes. There are FCC
regulations as well, as the frequencies used in computers interfere with
e.g. cell phones and other radio devices in the GHz range.
Sorry for any confusion, and remember, college physics textbooks aren't
too expensive and the wikipedia is free (and is rapidly becoming a
fabulous resource for physics as people write {\em great} articles for
it on advanced subjects.
rgb
> Isn't shielding 50Hz fields pretty expensive, or do we speak of other type
> of shielding here, so not getting rid of the electro magnetic field?
>> I tend to remember mu-metal is one of the metals that gets used, as it
> completely eliminates 50Hz fields. One of problems is it that not much of
> it gets made, so a square meter thin mu-metal is already a couple of
> hundreds of euro.
>> What other ways are there to get rid of the EMF?
>> I mean *solutions where with a tesla meter i can measure there is not much
> of the radiation left*, i do not mean methods where one must believe on the
> blue eyes of the salesman that it shields; i was threatened once by a
> courtcase when i concluded in the commission (high voltage power) publicly
> that a certain device wouldn't shield against EMF fields when a tesla meter
> still would measure the same amount of radiation after bringing the device
> to work under high voltage power cables (which have the tendency that more
> cancer underneath it happens, which doesn't necessarily mean it is the
> power cables themselves causing it, could be some other effect of course,
> just the increase in cancer is there).
>> In our search to shield off against such fields we only found mu-metal, but
> perhaps we missed some obvious solution that's getting used in electronics,
> or the prices quote we got were simply far too high (which happens a lot of
> course). I would really appreciate comments there, as i'm sure in mass
> market electronics such as clusters with huge fields and a lot of electric
> power,
> you really need to shield of equipment, so there must be many experts here.
>> Please note that general agreement seems to be that it is wise to avoid new
> situations where persons must nonstop live in a field above 0.4 microtesla.
>> How is this with electronic components?
>> Also note governments accept that peoples working conditions may be a lot
> worse than 0.4 microtesla.
>> Those building regulations are far newer than the outdated regulations from
> public healthcare. Some governments still use norms like 100 microtesla, a
> field you have basically when you put your head against a 400000 volt wire.
>> In general many electric devices have problems, or show disturbance
> regurarly with fields above 1-2 microtesla. That was however observed with
> amateuristic methods.
>> Vincent
>>> Jim Lux, IIRC, had a bunch to say on these latter points in previous
>> discussions to this point. He's the list's EE guru as much as anybody
>> and builds all sorts of "interesting" microclusters.
>>>> rgb
>>>>> Thankx³
>>>>>> --
>>>>>> Fernando.
>>>> --
>> Robert G. Brown http://www.phy.duke.edu/~rgb/>> Duke University Dept. of Physics, Box 90305
>> Durham, N.C. 27708-0305
>> Phone: 1-919-660-2567 Fax: 919-660-2525 email:rgb at phy.duke.edu>>>> _______________________________________________
>> Beowulf mailing list, Beowulf at beowulf.org>> To change your subscription (digest mode or unsubscribe) visit
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--
Robert G. Brown http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567 Fax: 919-660-2525 email:rgb at phy.duke.edu